Channel estimation in a CDMA wireless communication system

ABSTRACT

A method and apparatus for estimating channel conditions in a code-division multiple access (CDMA) communication system having a pilot signal and a traffic signal. The apparatus includes a pilot filter for generating channel estimates from the pilot signal and a circuit for reconstructing the traffic information bits after they have been decoded. A predictive channel estimation circuit generates predictive channel estimates from the original traffic signal demodulated by the reconstructed traffic information bits. A demodulator demodulates the traffic signal using the predictive channel estimates and the pilot signal channel estimates. By using the predictive channel estimates that contain signal energy from the traffic signal as a coherent reference, the channel conditions may be estimated more accurately than by using the pilot signal alone.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is aContinuation-in-Part/Continuation and claims priority to U.S. patentapplication Ser. No. 09/289,073 entitled “CHANNEL ESTIMATION IN A CDMAWIRELESS COMMUNICATION SYSTEM” filed Apr. 8, 1999, now U.S. Pat. No.6,452,917, assigned to the assignee hereof and hereby expresslyincorporated by reference herein.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to code-division multiple access (CDMA)wireless communication systems. More particularly, the present inventionrelates to a novel and improved method and apparatus for estimatingchannel conditions in a CDMA wireless communication system using decodeddata.

II. Description of the Related Art

In a wireless radiotelephone communication system, many userscommunicate over a wireless channel. Communication over the wirelesschannel can be one of a variety of multiple access techniques that allowa large number of users in a limited frequency spectrum. These multipleaccess techniques include time division multiple access (TDMA),frequency division multiple access (FDMA), and code division multipleaccess (CDMA).

The CDMA technique has many advantages. An exemplary CDMA system isdescribed in U.S. Pat. No. 4,901,307, entitled “Spread Spectrum MultipleAccess Communication System Using Satellite Or Terrestrial Repeaters”,issued Feb. 13, 1990, assigned to the assignee of the present invention,and incorporated herein by reference. An exemplary CDMA system isfurther described in U.S. Pat. No. 5,103,459, entitled “System AndMethod For Generating Signal Waveforms In A CDMA Cellular TelephoneSystem”, issued Apr. 7, 1992, assigned to the assignee of the presentinvention, and incorporated herein by reference.

In each of the above patents, the use of a forward-link (base station tomobile station) pilot signal is disclosed. In a typical CDMA wirelesscommunication system, such as that described in EIA/TIA IS-95, the pilotsignal is a “beacon” transmitting a constant zero symbol and spread withthe same pseudonoise (PN) sequences used by the traffic bearing signals.The pilot signal is typically covered with the all-zero Walsh sequence.During initial system acquisition, the mobile station searches throughPN offsets to locate a base station's pilot signal. Once it has acquiredthe pilot signal, it can then derive a stable phase and magnitudereference for coherent demodulation, such as that described in U.S. Pat.No. 5,764,687 entitled “Mobile Demodulator Architecture For A SpreadSpectrum Multiple Access Communication System,” issued Jun. 9, 1998,assigned to the assignee of the present invention, and incorporatedherein by reference.

A functional block diagram of a typical prior art forward link dataformatter as used by a CDMA base station is shown in FIG. 1. Data source102 may be, for example, a variable rate vocoder such as that describedin U.S. Pat. No. 5,657,420, entitled “Variable Rate Vocoder”, issuedAug. 8, 1997, assigned to the assignee of the present invention andincorporated herein by reference. Data source 102 generates trafficchannel information in the form of frames of digital data. CRC and tailbit generator 104 calculates and appends cyclic redundancy check (CRC)bits and tail bits to the frames generated by data source 102. The frameis then provided to encoder 106, which provides forward error correctioncoding, such as convolutional encoding, upon the frame as is known inthe art. The encoded symbols are provided to repetition generator 120,which repeats the reordered symbols to provide the appropriatemodulation symbol rate. The repeated symbols are then provided tointerleaver 108, which re-orders the symbols in accordance with apredetermined interleaver format. The repeated, interleaved symbolstream is then covered with one of a set of orthogonal Walsh sequencesin traffic Walsh coverer 122, and gain adjusted in gain element 124. Itshould be understood that other forward link data formatters are alsoknown in the art. For example, it is well known that the repetitiongenerator 120 may be placed after the interleaver 108.

Pilot signal generator 128 generates a pilot signal, which may be asequence of all ones. The pilot signal is then covered with the all-oneWalsh sequence and combined with the output of gain element 124 incombiner 136. The combined pilot channel and traffic channel data (whichmay be plus or minus ones) is then spread in PN spreader 138 using acomplex PN code generated by PN generator 140, and then transmitted byradio frequency transmitter 142 over antenna 144. A similar forward linkdata formatter is disclosed in co-pending U.S. Pat. No. 6,396,804,entitled “High Data Rate CDMA Wireless Communication System”, issued May28, 2002 assigned to the assignee of the present invention andincorporated by reference herein.

Other data formatting techniques also exist. For example, in thecdma2000 reverse link, the pilot signal is time-multiplexed with powercontrol commands. Additionally, in W-CDMA, the forward link usesdedicated pilot signals that are time-multiplexed with otherinformation.

FIG. 2 illustrates a functional block diagram of a typical prior artdata demodulator for use in a CDMA mobile station. Receiver 202 receivesand downconverts the signals transmitted by transmitter 142 of FIG. 1.The digital baseband output of receiver 202 is despread in PN despreader204 using the complex PN code generated PN generator 206, which is thesame complex PN code as that generated by PN generator 140 of FIG. 1.

The despread signal is then Walsh uncovered in traffic channel Walshuncoverer 208 using the same Walsh sequence as that of the trafficchannel Walsh coverer 122 of FIG. 1. The Walsh-uncovered chips are thenaccumulated into Walsh symbols in Walsh chip summer 210 and provided asa traffic channel signal to dot product circuit 212. In someapplications, an additional delay element (not shown) is introducedbetween Walsh chip summer 210 and dot product circuit 212 to account fordelays introduced by pilot filter 216. However, if pilot filter 216 is acausal filter, such a delay element (not shown) is not necessary. Thedot product circuit is also known as a “conjugate product” circuit. Itperforms the operation expressed mathematically by one of the followingequivalent forms: <a,b>=a□b=ab*.

The despread signal is also provided to Walsh chip summer 214 where theyare accumulated into Walsh symbols and provided to pilot filter 216 aspilot channel symbols. Note that since the pilot channel is covered withthe all-one Walsh sequence in Walsh coverer 134 of FIG. 1, a vacuousoperation, the corresponding uncoverer is also vacuous in operation.However, in the general case, the pilot signal may be uncovered usingany same Walsh sequence as is used to cover it. The pilot filter 216serves to reject the noise in the pilot symbols, providing a phase andscale reference for the dot product circuit 212.

Once per symbol, the dot product circuit 212 computes the component ofthe traffic channel signal in phase with the pilot channel signalgenerated by the pilot filter 216. As described in U.S. Pat. No.5,506,865, entitled “Pilot Carrier Dot Product Circuit”, issued Apr. 9,1996, assigned to the assignee of the present invention and incorporatedherein by reference, the dot product adjusts both the received signal'sphase and scale as needed for coherent demodulation.

The symbols output from dot product circuit 212 are de-interleaved inde-interleaver 218, using the same format used by interleaver 108 ofFIG. 1. The de-interleaved symbols are then decoded in decoder 220according to the error correcting codes employed by encoder 106 of FIG.1. The resulting decoded symbols are analyzed on a frame-by-frame basisby quality indicator CRC Check 222 to ensure that the frame was properlydecoded. If the frame was properly decoded, then that decoded frame isforwarded for further processing. Quality indicator CRC Check 222typically would examine the CRC portion of the frame, but may also useother frame quality indications such as Yamamoto metrics.

In a typical CDMA wireless communication system, such as that describedin EIA/TIA IS-95, the pilot signal energy may be less than the trafficsignal energy, depending on the data rate. Additionally, in recentlyproposed third-generation (3G) CDMA wireless communication systems, thepilot signal may not be transmitted continuously, but rather it mayshare time with a power control signal. For example, in a cdma2000system, the reverse link pilot signal shares time with a multiplexedpower control bit. In the W-CDMA system, the forward link dedicatedpilot channels are time-multiplexed. When the pilot signal is weak ornon-existent, coherent demodulation performance is degraded. Thus, aCDMA wireless communication system would benefit greatly from additionalsignal energy being used to provide a coherent channel reference and forestimation of the channel statistics.

SUMMARY OF THE INVENTION

The present invention is a novel and improved method and apparatus forestimating channel conditions in a code-division multiple access (CDMA)communication system having a pilot signal and a traffic signal. As usedherein, the term “traffic” signal is used to refer to a data-bearingsignal other than the pilot signal. For example, the traffic signal maycarry voice or data generated by one or more users, or it may carryoverhead information generated by the communication system.

The apparatus includes a pilot filter for generating pilot signalchannel estimates from the pilot signal and a circuit for reconstructingthe traffic information bits after they have been decoded. The originaltraffic signal is demodulated by the reconstructed traffic informationbits and is henceforth referred to as the “traffic-based channelreference”. A predictive channel estimation circuit generates predictivechannel estimates from the traffic-based channel reference and a delayedpilot signal. A demodulator demodulates the traffic signal using thepredictive channel estimates and the pilot-based channel estimates. Byusing the predictive channel estimates that contain signal energy fromthe traffic signal in addition to that from the pilot signal, thechannel conditions may be estimated more accurately.

The predictive channel estimation circuit includes a delay element fordelaying the pilot signal to match the timing of the reconstructedtraffic signal. A combiner combines the delayed pilot signal with thetraffic-based channel reference signal. The combiner may weight thetraffic-based channel reference signal relative to the delayed pilotsignal according to quality indicators of the reconstructed trafficchannel information bits. A predictive channel estimator generates thepredictive channel estimates from the combined delayed pilot signal andtraffic-based channel reference signal.

The circuit for generating the traffic-based channel reference signaldepends on the format used to generate the data signal waveform.However, in the preferred embodiment, it includes an encoder forencoding data symbols recovered from the traffic signal, and aninterleaver for interleaving the data symbols. Additionally, thedemodulator may include a controller for weighting the predictivechannel estimates according to a relative age of the predictive channelestimates.

In one application of the present invention, the controller alsodetermines whether channel statistics of the pilot signal channelestimate and the predictive channel estimate are correlated over apredetermined time period. The present invention also includes a methodfor estimating channel conditions. The method described herein mayperform by the apparatus described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a functional block diagram of a typical prior art forward linkdata formatter as used by a CDMA base station;

FIG. 2 is a functional block diagram of a typical prior art datademodulator for use in a CDMA mobile station;

FIG. 3 is an exemplary functional block diagram of the apparatus of thepresent invention; and

FIG. 4 is a flowchart of the method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 illustrates an exemplary functional block diagram of theapparatus of the present invention. Blocks labeled with like numerals asFIG. 2 correspond to similar elements as those described with referenceto FIG. 2, and perform similar functions. However, significantadditional functional blocks absent from FIG. 2 are illustrated in FIG.3 that form the basis of the present invention.

As in FIG. 2, the output of quality indicator CRC Check 222 is providedas user data for further processing to recover the information containedtherein. However, in contrast to the demodulator of FIG. 2, the input ofquality indicator CRC Check 222 is provided to encoder 302, whichre-encodes the data symbols using the same forward error correctioncoding techniques as that of encoder 106 of FIG. 1. The re-encodedsymbols from encoder 302 are then provided to interleaver 304, whichre-interleaves the symbols according to the interleaver format used byinterleaver 108 of FIG. 1. The output of interleaver 304 thus comprisesre-encoded, re-interleaved data symbols. If they have been decodedproperly as determined by quality indicator CRC Check 222, thesere-constructed data symbols output from interleaver 304 represent goodestimates of the signs of the data symbols being output by Walsh chipsummer 210.

In demodulator 305, these re-encoded, re-interleaved data symbols thatrepresent the reconstructed traffic channel information bits are used todemodulate the traffic channel symbols output from Walsh chip summer210, as delayed by delay element 307. The amount of delay introduced bydelay element 307 is designed to match the computing delay introduced bydot product circuit 212, deinterleaver 218, decoder 220, encoder 302,and interleaver 304. The resulting demodulated signal output fromdemodulator 305 to combiner 308 is referred to herein as thetraffic-based channel reference.

The output of Walsh chip summer 214 is delayed in delay element 306. Theamount of delay introduced by delay element 306 is designed to match thecomputational delays introduced by deinterleaver 218, decoder 220,encoder 302, interleaver 304, and demodulator 305 so that the pilotsignal output from delay element 306 is time-aligned with thetraffic-based channel reference output from demodulator 305. The pilotsymbols and the traffic-based channel reference are combined in combiner308 and provided to predictive channel estimator 310. Combiner 308combines the delayed pilot symbols and traffic-based channel referencesignal in a weighted fashion, according to quality indicators of thedecoded traffic frame, such quality indicator CRC Check 222. In a moregeneral case, other quality indicators, such as thesignal-to-noise-ratio estimates of the traffic symbols, can also beused. For example, if the traffic frame was properly decoded (and thusthe bits are known with high confidence), the traffic-based channelreference signal is given a higher weight than if the traffic frame wasnot properly decoded.

Predictive channel estimator 310 recovers the magnitude and phaseinformation of the channel reference from the combined pilot andtraffic-based channel reference output from combiner 308. In thepreferred embodiment, predictive channel estimator 310 is similar inconstruction to pilot filter 216, and may be a simple first-order IIRfilter or a FIR filter.

The pilot symbols output from pilot filter 216 and the combined pilotand traffic-based channel reference signal output from predictivechannel estimator 310 are received as inputs to controller 312.Controller 312 combines the pilot symbols output from pilot filter 216,and the combined pilot and traffic-based channel reference signal outputfrom predictive channel estimator 310 to dot product circuit 212 for usein the phase adjustment and scaling operations performed by dot productcircuit 212.

Controller 312 preferably uses a dynamic weighted combining techniquewhen combining the pilot symbols output from pilot filter 216 with thecombined pilot and traffic-based channel reference signal output frompredictive channel estimator 310. The weighted combining techniqueaccounts for the relative latency or “age” of the combined pilot andtraffic-based channel reference signal output from predictive channelestimator 310. Because of the time necessary to re-encode andre-interleave the reconstructed data symbols, their usefulness inestimating the channel conditions depends heavily on how quickly thechannel conditions are changing. If the channel conditions arerelatively slow changing as compared to the time required to reconstructthe traffic channel data symbols, then the reconstructed data symbolenergy is more useful than if the channel conditions are rapidlychanging. In either case, the predictive channel estimate output frompredictive channel estimator 310 becomes stale as time passes.

Thus, in the preferred embodiment, controller 312 weights the combinedpilot and traffic-based channel reference signal output from predictivechannel estimator 310 according to its age. For example, during thefirst part of a successive frame, when the predictive channel estimatehas just been calculated, the controller 312 weights it with arelatively high weighting factor. However, as time passes during theframe, the controller 312 weights it with successively smaller andsmaller weighting factors so that it contributes less and less to thechannel estimate being provided to dot product circuit 212. Whenpredictive channel estimator 310 calculates a new predictive channelestimate, controller 312 again weights it with a relatively highweighting factor, and so on. In this way, controller 312 accounts forthe “age” or latency of the predictive channel estimate.

In another aspect of the present invention, controller 312 also uses theadded energy of the predictive channel estimates from predictive channelestimator 310 to determine the channel statistics. For example, when themobile station containing the present invention is stationary, ortraveling at a slow speed, then the channel conditions are relativelystable over time. Conversely, when the mobile station containing thepresent invention is traveling at a relatively fast speed, the channelconditions will generally be uncorrelated over time.

Controller 312 samples the predictive channel estimate output frompredictive channel estimator 310 at different pairs of times with afixed time offset in each pair, and then takes the conjugate product ofthe pair samples to determine correlation. If the samples are highlycorrelated, then we infer that the channel conditions are relativelystable over time. If the two samples are not correlated, then we inferthat the channel conditions are changing significantly between thesampling time pairs separated by the fixed time offset. By using theadditional energy recovered from the traffic channel, controller 312 isable to more accurately determine the channel statistics than by usingonly the output of pilot filter 216.

The method of the present invention is illustrated in FIG. 4. At block400 channel estimates are generated from the current pilot signal. Thismay be performed, for example, by pilot filter 216 of FIG. 3. At block404, the traffic-based channel reference is generated from thereconstructed traffic information bits. This may include re-encoding byre-encoder 302, re-interleaving by interleaver 304, and demodulation bydemodulator 305. At block 406, predictive channel estimates aregenerated from the traffic-based channel reference. This may beaccomplished, for example, by predictive channel estimator 310. At block408, the received traffic signal is conjugate multiplied using thepredictive channel estimates from block 406 and the pilot-based channelestimates from block 402. This may be accomplished, for example, bycontroller 312 in conjunction with dot product circuit 212.

It should be noted that in the present invention, the additional energyfrom the predictive channel estimates are also used by controller 312 toassist in determining whether the channel statistics correlate over thepredetermined time frame. This additional energy in the predictivechannel estimate increases the accuracy of the determination. However,estimating channel statistics represents only one of the additionalapplications for the additional signal energy recovered from thepredictive channel estimate besides assisting in coherent demodulation.The teachings of the present invention are equally applicable to manyother applications where additional signal energy may be useful inmaking a more accurate determination of the channel conditions.

Thus, the present invention provides a method and apparatus for using atraffic-based channel reference for estimating the channel conditionsfor coherent demodulation. Additionally, the present invention asdescribed above may be used to assist in determining the correlation ofthe channel statistics over a predetermined time period. Thus, thepresent invention increases the accuracy of the channel estimate whenthe pilot signal energy is weak or non-existent.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

We claim:
 1. A method for estimating coherently demodulated channelconditions in a wireless communication system, comprising: generating afirst predictive channel estimate from a first reference channel;generating a second predictive channel estimate from a second channeland recovering energy from the second predictive channel estimate; andestimating statistics of the second channel using the first predictivechannel estimate, the second predictive channel estimate and therecovered energy from the second predictive channel estimate.
 2. Themethod of claim 1, wherein said generating a first predictive channelestimate is based on generating a first predictive channel estimate froma pilot channel and said generating a second predictive channel estimateis based on generating a second predictive channel estimate from atraffic channel.
 3. An apparatus for coherent demodulation, comprising:means for generating a first predictive channel estimate from a firstreference channel; means for generating a second predictive channelestimate from a second channel and recovering energy from the secondpredictive channel estimate; and means for estimating statistics of thesecond channel using the recovered energy from the second predictivechannel estimate.
 4. The apparatus of claim 3, wherein the firstreference channel is a pilot channel and the second channel is a trafficchannel.
 5. A mobile station for coherent demodulation, comprising: afilter for generating a first predictive channel estimate from a firstchannel; a predictive channel estimation circuit for generating a secondpredictive channel estimate from a second channel and recovering energyfrom the second predictive channel estimate; and a controller forestimating statistics of the second channel using the recovered energyfrom the second predictive channel estimate.
 6. The mobile station ofclaim 5, wherein the first channel is a pilot channel and the secondchannel is a traffic channel.
 7. A method, comprising: generating afirst predictive channel estimate from a first channel and a secondpredictive channel estimate from a second channel; recovering energyfrom the second predictive channel estimate; and estimating statisticsof the second channel using the energy recovered from the secondpredictive channel estimate.
 8. The method of claim 7, wherein saidgenerating a first predictive channel estimate is based on generating afirst predictive channel estimate from a pilot channel and saidgenerating a second predictive channel estimate is based on generating asecond predictive channel estimate from a traffic channel.